It is known that isopentene and other isoalkenes, or iso-olefins, produced by hydrocarbon cracking may be reacted with methanol and other C1-C4 lower aliphatic alcohols, or alkanol, over an acidic catalyst to provide tertiary amyl methyl ether (TAME) or the like. Generally, it is known that asymmetrical ethers having the formula (CH.sub.3).sub.3 C--O--R where R is a C1-C4 alkyl radical, are particularly useful as octane improvers for liquid fuels, especially gasoline.
MTBE, ethyl t-butyl ether (ETBE), tert-amyl methyl ether (TAME) and isopropyl t-butyl ether (IPTBE) are known to be high octane ethers. The article by J. D. Chase, et al., Oil and Gas Journal, Apr. 9, 1979, discusses the advantages one can achieve by using such materials to enhance gasoline octane.
The reaction of tertiary olefins with alkanol to produce alkyl tertiary alkyl ether is selective with respect to iso-olefins. Linear olefins are unreactive in the acid catalyzed reaction, even to the extent that it is known that the process can be utilized as a method to separate linear and iso-olefins. The typical feedstream of FCC C.sub.5 or C5+ crackate used to produce tertiary alkyl ethers in the prior art and containing normal pentenes and isopentenes utilizes only the tertiary olefin in etherification. This situation presents an exigent challenge to workers in the field to discover a technically and economically practical means to utilize linear olefins, particularly normal pentenes, in the manufacture of tertiary alkyl ethers.
European Patent 0026041 to Garwood, incorporated herein by reference, discloses a process for the restructuring of olefins in contact with zeolite catalyst to produce iso-olefins, followed by the conversion of iso-olefins to MTBE and TAME. The restructuring conditions comprise temperature between 204.degree. C. and 315.degree. C. and pressure below 5.1.times.10.sup.4 Pa. In European Patent 0247802 to Barri et al, it is taught that linear olefins can be restructured in contact with zeolite catalyst, including ZSM-23, to produce branched olefins. The restructuring conditions comprise temperature between 200.degree.-550.degree. C., pressure between 100 and 1000 MPa and WHSV between 1 and 100. It is taught that in the process 1-butene conversion exceeds 59 mole % with a selectivity to isobutene greater than 52 mole %.
It has been discovered that under certain conditions substantial improvements in the art of alkyl tert-alkyl ether production can be realized in a combination or integration of etherification and hydrocarbon conversion processes based upon zeolite type catalysis. In U.S. Pat. Nos. 4,788,365, 4,826,507 and 4,854,939 to M. N. Harandi and H. Owen novel processes are described for carrying out the production of MTBE and TAME wherein unreacted alcohol and light olefin components from the etherification reaction are converted to higher hydrocarbons in contact with zeolite catalyst. These patents are incorporated herein by reference. In these processes the etherification reaction is carried out using C.sub.4 + hydrocarbon feedstream rich in iso-olefins with the subsequent oligomerization of unreacted light olefins.
U.S. Pat. No. 4,605,787 to Chu et al., incorporated herein by reference, describes a process for the preparation of methyl tertiary butyl ether which comprises reacting isobutylene and methanol in the vapor phase in the presence of zeolite catalyst. Under the conditions described for the vapor phase etherification, side reactions, particularly the dimerization of isobutylene, are virtually eliminated. The reaction products are essentially MTBE and unreacted methanol and/or isobutylene.
U.S. Pat. No. 5,132,467 to Haag et al., incorporated herein by reference, describes a process for producing tertiary alkyl ethers wherein linear olefins are isomerized in the vapor phase to produce iso-olefin vapor which is etherified to form MTBE. Unreacted iso-olefin and/or linear olefin and product ether are separated by fractionation using fresh feed as a reflux stream to the fractionator and unreacted olefin components recycled.
U.S. Pat. No. 5,210,327 to Luebke et al., incorporated herein by reference, describes an etherification process and a process for isomerizing linear alkenes in the etherification effluent to isoalkenes. The process uses a distillation separation zone for the etherification effluent from which is taken an overhead rich in isopentane and a side cut comprising linear olefins which is cycled to a skeletal isomerization unit. The process provides for recycle of linear olefins while maintaining low mass flow through the isomerization zone by eliminating isopentane from the isomerization zone recycle feed. The process separates alkanol from the feed to the isomerization unit by interposing a water wash column to separate out methanol requiring further treatment of the feed to remove residual water and oxygenates.
It is an object of the present invention to provide a process for the etherification of linear olefins, particularly n-pentenes, to alkyl tertiary alkyl ether, particularly TAME.
It is another object of the invention to provide a process for the isomerization of linear olefins to iso-olefins followed by the etherification of the iso-olefins so formed to provide alkyl tert-alkyl ethers.
Yet another object of the instant invention is to provide an integrated process for the etherification of linear and branched olefin components of a hydrocarbon feedstream by combining sequential etherification reactions with linear olefin isomerization to produce alkyl tert-alkyl ether, particularly TAME, while minimizing mass flow to the etherification reactors by employing distillation to provide an alkanol-free, water-free olefin recycle stream to the isomerization reactor without using alkanol/water extraction of the stream and subsequent alkanol/water separation, thereby increasing the yield of ether on fresh feed with minimal expansion of the etherification reactors.
Still another objective of the present invention is to provide an integrated process for the plural stage etherification of linear and branched olefin components of a hydrocarbon feedstream by combining sequential etherification reactions with linear olefin isomerization to produce alkyl tert-alkyl ether, wherein the mole ratio of alkanol to iso-olefins in the first etherification stage is maintained at greater than 1 to provide higher conversion of iso-olefins to ether, while excess alkanol is reacted in subsequent etherification stage(s) with iso-olefins obtained from isomerization of linear olefins, to maximize ether production from a feed containing linear and iso-olefins.
Another objective of the present invention is to provide an integrated process for the plural stage etherification of linear and branched olefin components of a hydrocarbon feedstream by combining sequential etherification reactions with linear olefin isomerization to produce alkyl tert-alkyl ether, wherein the overhead effluent of the final etherification zone is separated by distillation to linear olefin-rich recycle stream and an overhead stream containing iso-pentane and only minor amounts of methanol and water, said overhead stream being suited to direct incorporation in a refinery gasoline pool.